Laminated and sheet filters have been known for a long time. These materials substantially consist of woven, knitted or needled fibers of different length which additionally have been reinforced with an adhesive or by mechanical means. As fiber material all sorts of materials come into practical consideration, either of natural or synthetic origin. However, these filters have the property in common--as they all have a more or less porous surface--of being undesirably clogged by the particles to be filtered out. Moreover, this effects an initially rapid throughput of filtrate which is perhaps not yet sufficiently thoroughly filtered, and a growingly insufficient throughput with increasingly clogged filter area.
Furthermore, the production of sheet structures by coagulation of a polymer solution has long been known. Moreover, it has been known to produce microporous sheet structures, e.g. from polyurethane solutions, according to the coagulation technique.
Therefore, it is an object of the invention to design the microporous and/or macroporous sheet structures obtained by coagulation such that, on the one hand, they may be used as membrane-type filters and, on the other hand, as genuine filters, e.g. as liquid filters.
It has been a further object of the invention to provide a process by means of which polyurethane solutions can be prepared in a very simple and reproducible way which, by means of the coagulation technique, give microporous and/or macroporous sheet structures of largely crosslinked polyurethane elastomers and which, after suitable after-treatment, constitute the filter materials of the invention.
Further objects of the present invention will become apparent from the following description.